For example, in the diagnosis of coronary arteries, the possibility of cardiac disease associated with plaques in the coronary blood vessel is diagnosed. When broken, the plaques in the coronary blood vessel block or extremely narrow the blood vessel, and induce serious ischemic heart disease. Thus, the coronary arteries are diagnosed. In the coronary artery diagnosis, a narrowed part of the coronary arteries is specified or a plaque in the coronary arteries which has not yet caused the narrowing of a blood vessel is specified from the CT image acquired by X-ray CT scanning. In the coronary artery diagnosis, the plaque amount in a coronary blood vessel wall is measured from a CT image acquired by X-ray CT scanning, and the risk of a coronary disease is diagnosed by the measured plaque amount. Under the present situation, a doctor observes the CT image, and manually extracts plaques, and then diagnoses the risk of a coronary disease in accordance with the amount of extracted plaques. The plaques are extracted by a technique which cuts the CT image by a preset threshold.